Small RNA inhibits infection by downy mildew pathogen Hyaloperonospora arabidopsidis.

Gene silencing exists in eukaryotic organisms as a conserved regulation of the gene expression mechanism. In general, small RNAs (sRNAs) are produced within the eukaryotic cells and incorporated into an RNA-induced silencing complex (RISC) within cells. However, exogenous sRNAs, once delivered into cells, can also silence target genes via the same RISC. Here, we explored this concept by targeting the Cellulose synthase A3 (CesA3) gene of Hyaloperonospora arabidopsidis (Hpa), the downy mildew pathogen of Arabidopsis thaliana. Hpa spore suspensions were mixed with sense or antisense sRNAs and inoculated onto susceptible Arabidopsis seedlings. While sense sRNAs had no obvious effect on Hpa pathogenicity, antisense sRNAs inhibited spore germination and hence infection. Such inhibition of infection was not race-specific, but dependent on the length and capping of sRNAs. Inhibition of infection by double stranded sRNA was more efficient than that observed with antisense sRNA. Thus, exogenous sRNA targeting conserved CesA3 could suppress Hpa infection in Arabidopsis, indicating the potential of this simple and efficient sRNA-based approach for deciphering gene functions in obligate biotrophic pathogens as well as for R-gene independent control of diseases in plants.

A Suppressor/Avirulence Gene Combination in Hyaloperonospora arabidopsidis Determines Race Specificity in Arabidopsis thaliana.

The pathosystem of Arabidopsis thaliana and diploid biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa) has been a model for investigating the molecular basis of Flor's gene-for-gene hypothesis. The isolates Hpa-Noks1 and Hpa-Cala2 are virulent on Arabidopsis accession RMX-A02 whilst an F1 generated from a cross between these two isolates was avirulent. The F2 progeny segregated 3,1 (avirulent, virulent), indicating a single major effect AVR locus in this pathogen. SNP-based linkage mapping confirmed a single AVR locus within a 14 kb map interval containing two genes encoding putative effectors. The Hpa-Cala2 allele of one gene, designated H. arabidopsidiscryptic1 (HAC1), encodes a protein with a signal peptide and an RxLR/dEER motif, and triggers a defense response in RMX-A02. The second gene is heterozygous in Hpa-Cala2. One allele, designated Suppressor ofHAC1Cala2 (S-HAC1Cala2 ) encodes a protein with a signal peptide and a dKEE motif with no RxLR motif; the other allele (s-hac1Cala2 ) encodes a protein with a signal peptide, a dEEE motif and is divergent in sequence from the S-HAC1Cala2 allele. In selfed progeny from Hpa-Cala2, dominant S-HAC1Cala2 allele carrying progeny correlates with virulence in RMX-A02, whereas homozygous recessive s-hac1Cala2 carrying progeny were avirulent. Genetic investigations suggested other heterozygous suppressor loci might exist in the Hpa-Cala2 genome.